A catalyst may be incorporated into an exhaust system of an internal combustion engine to convert hydrocarbons and NOx into CO2, N2, and H2O. The catalyst may begin its life cycle with a very high efficiency level, but the catalyst's efficiency may degrade as the catalyst reaches its lifecycle limit. If the catalyst degrades sufficiently, the vehicle in which the internal combustion engine resides may not meet a legislated emissions level. One way to determine whether or not the catalyst is degraded to a level where legislated emissions levels may not be met by the internal combustion engine is to compare a ratio of line lengths generated from oxygen sensor output voltage levels. In particular, an output voltage of an upstream oxygen sensor may be converted into a length of a line and an output voltage level of a downstream oxygen sensor may be converted into a length of a line. A ratio of the lines may then be a basis for determining whether or not a catalyst is degraded. However, if the downstream oxygen sensor output is degraded such that it exhibits characteristics of a low-pass filtered oxygen sensor output, then the ratio value used to determine catalyst degradation may be influenced such that the ratio value may not be relied upon for accurately assessing the presence or absence of catalyst degradation. Therefore, it may be desirable to provide a way of compensating for downstream oxygen sensor degradation in a way that allows the ratio value to be useful for assessing the presence or absence of catalyst degradation.
The inventors herein have recognized that oxygen sensor degradation may affect determined values of a catalyst index ratio and have developed an engine operating method, comprising: filtering output of an oxygen sensor located upstream of a catalyst in an exhaust system of an engine according to a response of an oxygen sensor located downstream of the catalyst; and adjusting an actuator responsive to the filtered output of the oxygen sensor.
By filtering the output of an upstream oxygen sensor according to a time constant of a downstream oxygen sensor, it may be possible to provide the technical result of improving evaluation of the presence or absence of catalyst degradation. Specifically, in one example, a response of an oxygen sensor exhibiting little degradation may be adjusted via a digital filter according to a time constant of a second oxygen sensor that may be exhibiting more significant degradation. The digital filter may more closely align response characteristics of the less degraded oxygen sensor with the response characteristics of the more degraded oxygen sensor so that an index ratio value that describes catalyst performance may be influenced more by catalyst performance than by oxygen sensor performance.
The present description may provide several advantages. Specifically, the approach may improve catalyst degradation assessments. Further, the approach may reduce false indications of catalyst degradation that may increase vehicle warranty costs. In addition, the approach may allow catalyst degradation to be evaluated against a single constant threshold value over a course of a vehicle lifetime so that indications of catalyst degradation and expected catalyst performance may be more reliable.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.